1. Field of the Disclosure
The present disclosure is related to a milling apparatus for milling casing exits and to perform other cutting operations in wellbores.
2. Background of the Art
Conventional cylindrical mills are commonly utilized for milling windows (or sections) in metal casings (such as pipes) placed in wellbores to provide exits for forming lateral wellbores and to perform other downhole cutting operations. Often, three mills (a window mill, a lower mill and an upper mill) are used on a bottomhole assembly (BHA) to perform the milling operations. Such mills generally include an enlarged pipe section (larger diameter section) that transitions to a smaller diameter pipe on both sides at a taper angle, typically 15°, with a small blending radius at both ends of the taper. Blades are welded over the tapered sections and the enlarged section. Although such tapers or tapered sections along with the small blending radiuses appear to provide a smooth transition between the two diameters to avoid stress concentration, the analysis and operational experience show a relatively high concentration of stress at such transitions. Additionally, the lower mill is the most highly stressed member of the bottomhole assembly (BHA). During milling operations, as the window mill moves down the ramp and laterally through the casing wall, the lower mill body is bent. High stress concentration occurs at the end of the blades, causing cracks to first appear near the ends of such blades. The lower mill is also subject to the substantial torque required to drive the mill, and to torsional impacts from the blades engaging (hitting) the side of the window and the cut slot. The torsional stress is sufficiently high to promote crack growth.
The disclosure herein provides a milling apparatus that addresses at least some of the above-described deficiencies of the mills.